[LLDB Test] Add '-link-objc-runtime' to 'swiftc' invocations when ObjC interop is required by the test.

[artemcm]

This ensures the Swift Driver will formulate a linker driver invocation that will pick up the ObjC runtime.

[artemcm]

swiftlang/swift-driver#1227
@swift-ci test macOS Platform

[artemcm]

@swift-ci smoke test

[kastiglione]

When can the driver self-determine that the objc runtime is needed? In other words, why do the tests require explicit opt-in?

[felipepiovezan]

Deferring to others here, as they are more familiar with the Swift components of LLDB

[artemcm]

When can the driver self-determine that the objc runtime is needed? In other words, why do the tests require explicit opt-in?

Linking ObjC runtime is a pretty heavyweight thing, as when we ask the linker driver (clang) to do so, it also will bring in Foundation. Which is not a reasonable default because it causes trivial programs to be very large. And there are cases where we want ObjC support, but no ObjC runtime linked, for example: if you're in a binary which Foundation itself links.

We already have many other Swift test cases specifying this, so it seems reasonable to expect this on tests that know they require it.

A little more context in swiftlang/swift-driver#1224.

[artemcm]

@swift-ci smoke test Linux platform

[artemcm]

@swift-ci test Linux platform